The invention relates to an operational amplifier (OA), particularly for signal amplification requiring high accuracy and extremely high speed which includes bandwidth, slew rate and settling time.
Two parts can be generally distinguished in every OA: the input and output stage. The input stage of the conventional OAs amplifies a differential input signal and converts it into a single signal. A high input impedance is common. However, a low impedance inverting input is provided in OAs using a current feedback for an excellent high frequency performance. An additional amplification is accomplished in the output stage which also has a low output impedance, short circuit protection, etc.
Conventional OAs comprise single-ended stages which cause a low, asymmetrical slew rate and a slow recovery from clipping.
For instance, a current source providing a fixed and mostly excessive current is employed in a stage driving the output transistors. Multiple gain stages cause cumulative phase shift, reduced phase margins, large signal transit time thru the OA and also require a limited slew rate to prevent a saturation.
The differential nature of the input signal, lost thru the conversion into the single signal in the input stage of the OA, must be regained in the output stage as the push-pull final stage is indispensable. A push-pull stage driving the output transistors requires a voltage shift and a double phase shift by 180 degrees. This contributes to a multiple pole roll-off and results in reduced output voltage swing, greatly impaired stability, instabilities with load fluctuations, etc.
The invention is intended to solve the above problems and therefore the object of the invention is to provide an OA having high accuracy, extremely high speed and a very low number of components. According to the invention an OA with an input stage providing a pair of output signals, and an adequate output stage coupled thereto solves these problems. Specifically, an OA according to the present invention includes a pair of transistors of opposite conductivity types, each having a pair of electrodes and a collector, means for applying an input signal of the OA to at least one of the electrodes of each transistor, means for coupling together at least one of the electrodes of each transistor; and a power amplifier means for amplifying signals appearing at the collectors of the transistors and providing an output signal of the OA.
In one embodiment the OA comprises a first and second transistors of opposite conductivity types, each having a first and second electrodes and a collector with the second electrode of each transistor being coupled to one of the electrodes of the other transistor, a voltage follower having an input, being coupled to the first electrode of the second transistor and exhibiting such an offset voltage that the offset voltage appearing between its input and the first electrode of the first transistor is substantially equal zero, wherein an input signal of the OA is applied to the first electrode of the first transistor and the input of the voltage follower, a pair of means for biasing the first and second transistors coupled separately to the collectors thereof and a power amplifier means for amplifying signals appearing at the collectors of the first and second transistors and providing an output signal of the OA.
In another embodiment the OA comprises a first and second transistors of opposite conductivity types, each having a first and second electrodes, and a collector, a first and second voltage followers each having an input and being coupled to the second electrode of the first and second transistors respectively, wherein an input signal of the OA is applied to the first electrode of the first transistor and the input of the first voltage follower, further wherein the first electrode of the second transistor and the input of the second voltage follower are coupled in either order to the first electrode of the first transistor and the input of the first voltage follower, a pair of means for biasing the first and second transistors coupled separately to the collectors thereof, and a power amplifier means for amplifying signals appearing at the collectors of the first and second transistors and providing an output signal of the OA.